This invention relates to apparatus for monitoring a-c variables of predetermined periodic waveform with constant amplitude, frequency and phase in general, and more particularly, to improved, quick acting monitoring apparatus.
For monitoring alternating (a-c) variables, an actual value a-c voltage which is taken off from a suitable measuring pickup or measuring transducer, can be rectified and smoothed as well as possible. For instance, by using a Graetz (full-wave) bridge as the rectifier circuit, one obtains a rectified voltage with two pulses from the actual value a-c voltage. The smoothing member, i.e., filter, following the rectifier circuit, usually a capcitor, has a smoothing time constant which is long relative to the period of the a-c variable monitored in order to smooth the voltage rectified by the rectifier circuit sufficiently. The rectified and smoothed voltage has a rectified value which corresponds, with a high resistance termination, essential to the ideal no load d-c voltage of the rectifier circuit. The rectified value of the actual value a-c voltage can be compared with a very constant reference value d-c voltage or fed to a limit indicator with a predetermined response threshold. However, the rectified value of the actual value a-c voltage only follows the monitored a-c variable with a delay caused by the smoothing time constant of the smoothing member, so that, for instance, rapid detection of voltage breaks or voltage spikes and of phase changes and frequency changes in the monitored a-c variable is not possible.